The move comes as Venezuela, also under US pressure, begins releasing political prisoners.
The development in Nicaragua reflects the degree of pressure some left-wing governments in Latin America are under to appease US demands, especially following last week's stunning capture of Venezuela's president, Nicolas Maduro.
Maduro, who said he was kidnapped, faces US federal charges of narco-terrorism and drug trafficking.
In a statement on Saturday, local time, the government of Nicaraguan President Daniel Ortega announced the inmates' release without confirming the exact number of people freed or whether they had been detained for political reasons.
It's unclear whether those released will be restricted to house arrest.
A human rights NGO that tracks political prisoners in Nicaragua identified 19 people released on Saturday.
Opposition leader and former prisoner Ana Margarita Vijil, head of the UNAMOS political movement, told Reuters those released "are political prisoners, and several of them are our friends".
Vijil said she did not know the number of those involved but said they include former mayor Oscar Gadea and Evangelical pastor Rudy Palacios along with four of his relatives.
Palacios was detained in July after criticising the government for human rights violations. He had also supported demonstrators who took to the streets in 2018 to demand Ortega's ouster.
Ortega responded to the protests with repression that left at least 350 dead and hundreds detained. He and his wife, Rosario Murillo, control virtually every aspect of the government, including the armed forces and the judiciary.
On Friday, the US embassy in Nicaragua praised the release of opposition figures in Venezuela and called on Ortega's government to follow in their steps.
"In Nicaragua, more than 60 people remain unjustly detained or missing, including pastors, religious workers, the sick, and the elderly. Peace is only possible with freedom!," the embassy tweeted.
Liberales Nicaragua, a coalition of opposition groups, praised the inmates' release in a statement.
"We have no doubt that such a decision is the result of political pressure exerted by the US government on the dictatorship and of the political chess moves triggered by events in Venezuela."
